Once you upgraded to Windows 7, the only way to get back to Vista is through new OS installation. You need a Vista installation disc in order to install the OS from scratch. Even if you use a system image, you still need an application to restore that image to the system partition. Since there could be issues, I recommend formatting the partition drive to install the OS, then use a backup application to create a system image to prevent situations like these in the future. For backup procedure, use Acronis TrueImage.